Method for the production of hybrid spherical molded bodies from soluble polymers

ABSTRACT

The invention relates to a method for producing hybrid spherical molded bodies from soluble polymers and at least one embedded additive in that an additive-loaded polymer solution is dispersed in an inert solvent, and said dispersion process is carried out at a reduced pressure, the resulting particle dispersion is cooled below the solidification point of the polymer solution, the stabilized particles of the polymer solution are separated from the inert solvent, the separated particles of the polymer solution are precipitated in a solvent coagulating the polymer, the solvent-moistened polymer particles are subjected to a drying process until maximal densification is obtained, and the resulting particles that are made of polymer and additive are sintered by way of a thermal treatment to yield porous and/or highly condensed molded bodies. Resulting therefrom are highly stable molded bodies which do not sinter together during sintering process.

The method according to the invention relates to the production ofregular cellulose and/or ceramic, preferably spherical molded bodies ina range of particle sizes of from 1 μm-1000 μm by dispersing solutionsof cellulose in organic solvents in liquid, inert carries media and insubjecting the resulting molded bodies to a thermal treatment.

PRIOR ART

Small-dimensioned particles in all classes of sizes have acquired a firmplace in innumerous industrial applications in the last decades. Today agreat number of inorganic materials is commercially available, apartfrom a plurality of polymer matrices. The multitude of possibleapplications in industrial processes of biotechnology, separationtechniques and modern reaction technique permanently opens new fieldsfor these materials.

Not only the kind of the substances used but also their form arechallenging further developments. So there is an increasing demand forideal spherical particles, which may satisfy the demand for, forexample, an optimal packaging in fillings or a uniform whirling behaviorin whirling layers. Moreover, as concerns large dimensioned columns, thepressure constancy is of decisive importance. The manufacture ofsmall-dimensioned spherical inorganic particles is carried out by themost diverse manufacturing methods. Basically, there are two mainstreamsdominating: on the one hand, the bead formation via dispersing of filledpolymer solutions, on the other hand, the processing of sols and gelswith a subsequent calcinations. So, for example, according to EP 1108698and EP 0353669, ceramic powder, bound to polymers, is worked intoaqueous mash and this being dispersed with a liquid which is immisciblewith water. Further applications (U.S. Pat. No. 5,384,290, EP 0300543,EP 0369638) describe the bead formation by use of foamable pre-polymers;the beads themselves after formation are used for solidifying thestructure. Furthermore, a great number of patents based on thesol-gel-techniques gives evidence of the importance of this on thesol-gel-techniques gives evidence of the importance of this technologyfor producing ceramic molded bodies (for example, U.S. Pat. No.5,064,783, EP 0745557).

As to a fidelity of shape and the simplicity of processing theconventional standard forming technologies via, for example, polymericbound suspensions of particles are up-to-now no practicable alternative,since the particle size is strongly limited towards smaller sizes due toprocess inherent factors. Further known polymer forming technologiessuch as melting/solidifying (polyamide) or chemicalmodification/regeneration (viscose process, carbamat process) do notpermit such high a loading of additives which is required for aform-stable processing. Sol-gel technologies cannot ensure a sufficientform stability in the aimed at range of particle diameters since herebythe transition from the stabilized gel state into, for example, theoxidic ceramic is accompanied by high bulk losses and resulting highporosity. With respect to simplicity and efficiency the Lyocelltechnology has proven to be extraordinarily suitable. As alreadydescribed in DE 197 55 352 C1 and DE 197 55 353 C1 technologies such asthe dispersing of cellulose solutions in inert carrier material, whichdo not precipitate cellulose, or the beam cutting of cellulose solutionswill yield spherical particles over a wide range of particle sizes.

Furthermore, in DE 199 10 012 C1 (WO 00 538 33) there was set out thatsolutions of cellulose in N-methylmorpholin-N-oxid-monohydrate has avery large admission capacity for foreign matter. Thereby load rates ofcellulose/additive of more than 1:5 can be realized with additives ofhigh density. Such solutions still show a very good capability forfilament formation even with loads being a multiple of the celluloseweight and, hence, can be extruded to elongated molded bodies (fibers,filaments) or molded to plastic foils without problems. The forming ofmaterials of different kind which themselves, at low temperatures, donot exhibit the possibility of deformation (metals, for example) or donot have such material properties which would permit a self-deformationdue to missing plasticity (for example, low-molecular crystallinecompounds, ceramic powders) are, however, not described in the prior artmentioned.

OBJECT OF THE INVENTION

It is an object of the invention to provide a method which permits sucha (de)formation of various materials and thereby solves, above all, theproblem of stability at thermal treatment, since too low a packagedensity can result in a structural break-down, particularly afterpyrolysis of the binder. Furthermore, density gradients, which are dueto an inhomogeneous package of particles and included air-bubbles,result in considerable variations in strength of the sintered finalproducts und are potential sites of fractures, particularly inhigh-strength ceramic molded bodies. Closely connected therewith is theobject so ensure that the molded particles will not be subjected in thesubsequent processing steps to further variations in form, apart fromthe beginning shrinkage.

This object is realized in that a soluble polymer from the group of thepolysaccharides, preferably cellulose, together with the respectiveadditives, organic or inorganic, low-molecular or high-molecular,thermally stable or decomposable and capable of being sintered such as,for example, ceramic powder, is dissolved (dispersed) inN-methylmorpholin-N-oxid mono-hydrate, which is immiscible with thesolution, does not affect the cellulose in precipitating the same anddoes not undergo a chemical interaction with the cellulose and theadditives and all that, according to the invention, being carried outunder a reduced pressure, subsequently the resulting dispersion iscooled, whereby the solution drops solidify, the solidifying solutiondrops are entirely separated from the carrier medium, the polymersolution being formed in this manner is brought into a precipitatingmedium, whereby the pre-shaped spherical form of the solution drops ispermanently stabilized, the resulting highly swelled particles are, ifdesired, impregnated with compounds being solved in water or in organicsolvents, and, according to the invention, the drying of thesolvent-moistened polymer particles is continued until said particlesare maximally condensed, and/or the resulting filled particles aresubjected to a thermal treatment.

In the method according to the invention the advantages of the abovedescribed bead formation by Lyocell technology and the possibility ofhighly overloading the cellulose solutions are combined with oneanother. There was surprisingly found that also highly filled solutionswith loading rates of a cellulose-additive-ratio of 1:7 (parts ofweight) can be formed to spherical molded bodies of high stability, whenan extrusion of the solution by annular nozzles or slit nozzles meetsits ultimate limits. Furthermore there was found that the molded bodiesmaintain their form stability in the subsequent steps of processing,particularly in the thermal treatment, will stand the burning out of thesupporting cellulose matrix (release) even in multiple layers under theeffect of the own weight of the feed and subsequently they can bedensified nearly up to the theoretical density of the respectiveadditive by a sintering process.

As already described in DE 199 10 012 C1 the required cellulosesolutions will be produced from air-dry cellulose, an aqueous solutionof N-methylmorpholin-N-oxid and the respective additive. All suchsubstances are suitable for additives that are mechanicallydisintegrated fine enough or will dissolve in the course of the processof producing the solution, that do not undergo any interaction with theorganic solvent, the cellulose or the water, that will stand theexclusively aqueous processing without any changes and will not beexhausted to much by the extraction processes, and will have asufficient sintering activity in the case of application of ceramicmolded bodies. In the case of inorganic filled molded bodies, a loadingratio can be set of cellulose:additive=1:1 up to 1:8, preferably between1:3 and 1:6 in dependence on the density of the material used and therespective application. This ratio can be varied in as much as it ispermitted by the stability of the molded bodies after their release, 15weight-% cellulose being the supporting matrix for 85 weight-% additivesat a ratio of, for example, 6 parts additive to 1 part cellulose. Thus,the density of the molded bodies can be set in a simple manner via thedegree of loading. High degrees of loading will result in densitieswhich will be scarcely achieved by other non-pressure ceramic formationprocesses at formations bound to polymers (slip molding). With lowloadings, on the other hand, there will be formed an open-pore meshafter the burning out of the supporting cellulose, whereby the mesh canbe sintered to porous molded bodies with the porosity beingpre-settable. Furthermore, there are mixtures of additives possiblewhich, for example, improve the sintering activities, undergo reactionsduring the thermal treatment, for example, the formation of catalyticactive metal layers or themselves react with the additives, for example,a formation of mixed phases. When adhering to the above mentionedconditions, it is also possible to work in compounds which only formstable phases when under thermal stress or will be anchored generally inthe polymer mesh for a later application without that a pyrolysis of thesupporting polymers is carried out.

The viscosity of the cellulose solution is very strongly affected by theconcentration of the cellulose and by the additives. In the methodaccording to the invention there are cellulose solutions used which haveconcentrations of cellulose of from 1.5 to 15 weight-%, preferably 3-9weight-%. Additionally, there is a strong increase in viscosity at highdegrees of loading, the increase in viscosity having a decisiveinfluence on the subsequent dispersing and on the spectrum of particlesobtained therewith. In the method according to the invention, the loadedcellulose solutions are molded in a solvent at increased temperatures tospherical structures. To obtain this, all liquids are suitable which donot result in an immediate precipitation of the cellulose (water,alcohol) or to an extraction of the organic solvents (DMF, acetic ester)and which can form a stable dispersion with the solution. Mineral oil,silicone oil, native vegetable oil, waxes and paraffin as well asmixtures of biphenyl and biphenyl ether have proven useful.

The solution drops are finely dispersed into the carrier medium byapplying mechanical energy, taking on an ideal spherical form. Suchforms of stirrers are suited thereto, which exhibit a low shearingaction in the stirring zone, but ensure a fine vortex bale formation,for example, propeller stirrers, blade stirrers, and horseshoe stirrers.There was surprisingly found, that even extremely viscous solutions hadsafely be transformed into beads by applying low mechanical energy andhad been kept in stable dispersions. The speed of rotation to be useddepends on the density of the solution und of the carrier medium and liein a range of between 100 and 10.000 rpm, preferably between 250 and3000 rpm. The ratio of mixture (loading of the carrier medium withcellulose solution) can be between 0.01 and 0.5, preferably between0.07-0.4. The dispersing temperature should be above the melting pointof the cellulose solution, but it could fall below the same with shorttime dispersing. The dispersing will preferably be carried out at 75 to100° C. The spherical particles will be obtained in a range sizesbetween ?10 and 1000 μm, preferably ?50-500 μm, depending on thestirring parameters, the pressure in the stirring vessel, the ratio ofmixture, the temperature, and the properties (loading, viscosity) of thecellulose solution. As it generally known, there happens in spite ofcountermeasures a drag-in of gas into the dispersion with numerousdispersing processes due to surface turbulences. According to theinvention this problem is avoided in operating at a reduced pressure.Thereby the cellulose solution as well as the carrier medium iscontinuously degassed before and during the dispersion so that no gasbubbles will enter the beads during the solidification phase.

When the dispersion is then, under stirring, cooled below thesolidification temperature of the cellulose solution, the spherical formis maintained at first. After separating the carrier medium, in thesimplest way by decantation and filtration, the form stabilization iscarried out by coagulation in a precipitating agent, preferably water,which, if required, is provided with additives. In order to modify theprecipitation inferior alcohols may be used. The filled cellulose beadsproduced in such a manner represent highly soaked structures ofamorphous cellulose, wherein the additives are finely distributed. Inthis processing step, and if required, an additional impregnation withmodifying agents can also be carried out. This will be advantageousparticularly when the respective material is water-soluble or otherwiseincompatible with the used amine solvent. Thus considerable amounts ofcompounds can be embedded in the cellulose mesh afterwards, or additivesthat are already present may be modified. Here, in particular, it isthought of an impregnation with metallic salt solutions or sols, butalso, in the simplest case, of an elution with solved organic compounds,if required after an exchange of the solvent.

In the subsequent drying step, a strong enrichment of additives isachieved and, due to the beginning shrinkage of the cellulose, to aconsiderable densification. Surprisingly there was found that in thecourse of this shrinkage process the form of the particles, which hadbeen set in the dispersing process, was maintained. In addition to theconsolidation of form and densification, even water-soluble additivesare now securely bound within the now crystalline cellulose matrix,which is a considerable advantage in, for example, controlled-releaseapplications of agents. This phenomenon results therefrom that thetransition from the amorphous to the crystalline cellulose isaccompanied by an irreversible crystallization, whereby the compoundswhen having been embedded will now be stronger bound to the mesh thanwould have been obtained by an impregnation of, for example, soakedcellulose.

The separation of the spherical particles will be achieved either by wetsifting, more preferably by dry sifting, or by air-separation into grainsizes of defined composition. By selecting the stirring parameters theobtained particle spectrum can be narrowly distributed so that there areonly minor efforts necessary for separation.

The obtained spherical molded bodies are composites of preferablycellulose with additives and are ready for further applications andprocessing, respectively, in this form.

They will be substantially used as green bodies for the manufacture ofceramic beads which can find applications as grinding granules,chromatographic carrier media, catalyst supports, and beads beingcapable of heat sterilization in medical applications. Moreover,additive-loaded pure cellulose beads have a great potential for use in,for example, controlled-release applications, when releasing agents inthe medical field.

The invention will be explained in more detail by the followingexamples.

EXAMPLES Example 1

Into 500 g of a 50% aqueous solution of N-methylmorpholin-N-oxid 20 g ofa cotton-Linters-pulp (DP 477) and 100 g aluminum-oxide (d₅₀=0.7 °m) aregiven. Under intensive kneading water is distilled off in vacuum at 50millibar at 85° C. as long until a homogenous viscous solution results.The still liquid cellulose solution is coated with 927 g viscousparaffin oil (100 mPa.s) and stirred with a propeller stirrer for 5 minat 1500 rpm. Then a quick cooling is carried out under constant stirringuntil the solution, which is transformed into beads, solidifies. Afterdepositing decantation is performed, superfluous oil is sucked off andsubsequently precipitation is carried out in warm water. After severalwashing treatments with hot water still adherent residues of oil will beremoved by hot extraction with ?tertiary butanol and subsequently mildlydried. There will result highly filled beads with a portion of 83%filling material and in a diameter range of 100-500 μm. The burning-outof the cellulose and a subsequent sintering at 1450° C. will yield denseand hard corundum beads.

Example 2

11.3 kg silicon oil (250 mPa.s) are received in a heatable stirringvessel at ambience. There into 2 kg of a solid 8 weight-% cellulosesolution are given which is filled in a ratio of cellulose:titaniumdioxide=1:8. The system is air-tightly sealed and heated to 90° C. Afterthe solution is entirely melted, it will be stirred for 30 min. at aspeed of rotation of 800 rpm at a pressure of 0.1 mbar and, afterturning off the heating, the speed of rotation is stepwise increased to2000 rpm until solidification. The obtained beads are filtered off,precipitated in hot water, and washed three times with hot water.Residuals of silicon oil are removed by washing with ethanol. Thesubsequent drying yields TiO₂-filled beads having a titanium dioxidepercentage of 89%. The diameters of the particles lie between 20 and 150μm. The thermal treatment at 1800° C. results in ceramic beads oftitanium dioxide.

Example 3

5 kg of an eutectic mixture consisting of biphenyl ether and biphenylare filled up with 2.3 kg of a solution consisting of 102 g cellulose(Cellunier F), 1950 g N-methylmorpholin-N-oxid monohydrate and 255 gboron carbide (d₅₀=1 μm) and heated to 75° C. The melted solution isdispersed by a 4-blade stirrer and an ultrasonic horn within 10 min. at1 mbar and, after turning off the ultrasonic transmitter, quickly cooledto 20° C. under stirring, whereby a solidification of the moldedparticles starts. After depositing they are filtered at 40° C. and thefilter cake is washed again with isopropanol. The precipitation iscarried out in warm water and after several extractions of any stilladhering solvent the molded beads can be dried. After a non-pressuresintering at 1800° C. dark colored hard beads of boron carbide willform.

Example 4

By way of a piston spinning device, 513 g of a cellulose solution whichconsists of 75% N-methylmorpholin-N-oxid monohydrate, 8.5% cellulose,and 16.5% zirconium oxide, are injected within 30 min. into 2 kg ofhighly viscous paraffin oil, which is thermostated to 25° C., underheavy stirring (3000 rpm) and under a pressure of 0.5 mbar. Thereby asolidification of the forming spherical particles takes place, which aresucked off and are coagulated in warm water. After repeated washing withwater and ethanol a mild drying is carried out. Beads of cellulose withembedded zirconium oxide particles in a range of sizes of from 100 to700 μm will be obtained. After debinding and sintering hard andpressure-stable spheres of zirconium oxide will result.

Example 5

Spherical molded bodies within a range of diameter of ?50-250 μm areproduced under a pressure of 5 mbar and stirring in warmed paraffin oilfrom a 7.5 ?weight-% solution of cellulose, which contains one weightpercentage cellulose to one weight percentage aluminum oxide. Thesebodies are separated from the carrier medium, coagulated and releasedfrom the paraffin residuals by a repeated washing and final extraction.The still moist beads are placed for 30 min. in a 5 weight-% solution ofhexa-chloroplatinic acid, filtered off and dried. After debinding andsintering under atmospherical air porous beads of aluminum oxide withembedded finely distributed platinum oxide are obtained.

Example 6

Cellulose beads were made according to DE 197 55 352 C1 from a 6weight-percent cellulose solution, which contains 10 weight-percentglucose, under a pressure of 0.1 millibar. After precipitation andextraction the moist beads are treated for 10 min. with a concentratedsolution of nickel sulfate and subsequently dried. The thermal treatmentis carried out under exclusion of oxygen in an inert gas stream. Therebyporous particles are formed with embedded finely distributed parts ofnickel oxide.

Example 7

200 g of a 7.5 weight-percent cellulose solution, which contains 30weight-percent of starch, are melted in 2300 g highly viscous paraffinoil at 80° C. and subsequently distributed into fine solution drops by apropeller stirrer for 15 min at a pressure of 0.01 millibar. Thedeposited suspension of particles is separated, de-oiled and coagulatedin warm water. After the extraction with ?tertiary butanol the moistcellulose beads will be immersed in a 10 weight-percent solution ofacetosalicyclic acid in aqueous ethanol for 120 min., filtered off andsubsequently dried. Form stable cellulose beads with embeddedacetosalicyclic acid will result.

1. Method for producing hybrid spherical molded bodies from solublepolymers from the group of polysaccharide (starch, dextran), preferablycellulose and at least one embedded additive, in that the additiveloaded polymer solution is dispersed in an inert solvent, the resultingparticle dispersion is cooled to a temperature below the solidificationpoint of the polymer solution, the stabilized particles of the polymersolution are separated from the inert solvent, the separated particlesof the polymer solution are precipitated in a solvent coagulating thepolymer, characterized in that a) the dispersing process is carried outat a reduced pressure, b) the solvent-moistened polymer particles aresubjected to a drying process until the maximal densification isobtained, and c) the formed particles out of polymer and additive aresintered to porous and/or highly condensed molded bodies under thermaltreatment.
 2. Method as claimed in claim 1, characterized in that thedispersion process is carried out in vacuum between 10⁻⁴ and 100millibar, preferably between 0.01 and 1 millibar.
 3. Method as claimedin claims 1 and 2, characterized in that the stirring process isperformed entirely or partially under vacuum.
 4. Method as claimed inone of the claims 1 to 3, characterized in that the polymer is solved inan amino solvent, preferably in N-methylmorpholin-N-oxid.
 5. Method asclaimed in one of the claims 1 to 4, characterized in that the polymersolution contains at least one additive in the range of 0.01 to 1000weight-%, preferably between 5 and 700 weight-%, related to the part ofthe polymer.
 6. Method as claimed in one of the claims 1 to 5,characterized in that the additive/s is/are heavily soluble or insolublein the inert solvent.
 7. Method as claimed in one of the claims 1 to 6,characterized in that the additive/s is/are organic or inorganic,low-molecular or high-molecular, thermally stable or decomposable andcapable of sintering such as, for example, ceramic powder.
 8. Method asclaimed in one of the claims 1 to 7, characterized in that theadditive/s has/have a size of particles of from 10 μm to 1000 μm,preferably 50 μm to 5 μm.
 9. Method as claimed in one of the claims 1 to8, characterized in that the inert solvent is from the group of thesaturated aliphatic or unsaturated aromatic hydrocarbon, from thesaturated and unsaturated fatty acid esters and linear as well as cyclicpolysiloxane.
 10. Method as claimed in claims 1 and 9, characterized inthat stirrer systems in the range of rotation numbers of between 10 and20.000 rpm are employed for the energy input when forming the sphericalparticles from the solid loaded polymer solution.
 11. Method as claimedin one of the claims 1 to 10, characterized in that the dispersion iscooled down to a temperature of from 60 to 10° C., preferably 0 to 10°C. for stabilizing the spherical molded bodies.
 12. Method as claimed inone of the claims 1 to 11, characterized in that the coagulating mediumis preferably water.
 12. Method as claimed in one of the claims 1 to 11,characterized in that the coagulating medium is preferably water. 13.Method as claimed in one of the claims 1 to 12, characterized in thatthe supporting polymer matrix is pyrolized without residue at thethermal treatment.
 14. Method as claimed in at least one of the claims 1to 13, characterized in that the porous or dense molded bodies caninclude one substance or more with inherent functional properties suchas, for example, electric, magnetic, or catalytic activities.
 15. Methodas claimed in at least one of the claims 1 to 14, characterized in thata second or further substance/s is/are worked into the polymer solutionprior to dispersing.
 16. Method as claimed in at least one of the claims1 to 15, characterized in that the second or further substance/s is/areafterwards worked into the still solution-moistened spherical moldedbodies.